Hi Suzie
Bad luck about London. There is a thread on this year's Shakespeare Marathon earlier in this forum. It's a mid-sized race which offers a different, much more rural atmosphere than London.
Most of the two-lap Stratford course is very flat indeed, along an old railway line now called The Greenway, but there is one short stiff incline and one longer hill on each loop. The race starts at 1pm, so it can get quite warm and it's not an easy PB course. About 450 runners completed the marathon last year, a number almost 50% up over the past two years, probably as the popularity of London has increasingly exceeded demand. There is a half marathon with about 1500 runners on the same course, so plenty of company on lap one. Much sparser on lap 2.
Ian and I both ran this race last year. You can find Ian's report at:
http://www.ianpainter.com/shakespeare_marathon_2003.htm ,
whilst my own version lurks in Andy's 'Writing About Running' section of the Running Commentary website.
It's a very well-organised, friendly and scenic event, and I'd definitely consider doing the half marathon in future, if not the marathon again here just yet.